The art of surveying, or range finding, involves the determination of unknown positions, surfaces or volumes of objects using measurements of angles and distances. In order to carry out these measurements, an optical surveying instrument or geodetic instrument often comprises an electronic distance measuring (EDM) device which may be integrated in a so-called total station or a scanner. A distance measuring total station combines electronic and optical components and is furthermore in general provided with a computer or control unit with writeable information for controlling the measurements to be performed and for storing data obtained during the measurements. Preferably, the total station calculates the position of a target in a fixed ground-based coordinate system. A more detailed description of such a total station may be found in, e.g., WO 2004/057269 by the same applicant.
In conventional EDM, a light beam is emitted as a light pulse towards a target, and light reflected against the target is subsequently detected at the optical instrument, such as a total station. Processing of the detected signal enables determination of a distance to the target by means of, e.g., time-of-flight (TOF) or phase modulation techniques. Using a TOF technique, the time of flight of a light pulse that travels from the surveying instrument (the EDM device) to a target, is reflected at the target and returns to the surveying instrument (the EDM device) is measured, on the basis of which the distance may be calculated. The power loss of the received signal determines the maximum possible range. Using a phase modulation technique, light modulated at different frequencies is emitted from the surveying instrument to the target, whereby reflected light pulses are detected and the distance is calculated based on the phase difference between emitted and received pulses.
In a conventional scanner, the light beam may be guided over a number of positions of interest at the surface of the target using a beam steering function. A light pulse is emitted towards each of the positions of interest and the light pulse that is reflected from each one of these positions is detected in order to determine the distance to each one of these positions. For example, using a LIDAR (Light Detection and Ranging) scanner, or an industrial scanner, properties of scattered light may be measured to find range and/or other information of a distant target. In general, the distance to an object or surface is determined using laser pulses or a phase modulation technique such as mentioned above.
The detected signal representative of a reflected light pulse, i.e. the return signal, may have a wide dynamic range. In other words, the strength or power (or the amplitude) of the return signal may vary significantly from one position to another. Variations of the strength of the return signal may be explained by, e.g., differences in reflectivity between different positions at the surface of the target and/or differences in the topography of the target. As a result, distances determined on the basis of a return signal having a too high or too low power may not be accurate because of difficulties in handling a wide dynamic range at the measuring device. The detected signal may, e.g., be saturated or subject to excessive noise or interference.
Thus, the amplitude of the return signal at the input of the readout electronics (i.e. components employed to register the reflected optical pulses) at the measuring device, such as an EDM device or a scanner, must be limited in order to avoid saturation of the readout electronics. For example, the dynamic range of high-speed A/D converters is normally limited to about 25 dB, while the variation of the amplitude of a signal detected at the measuring device may be considerably higher, e.g., about 60 dB-100 dB or higher.
In some conventional measuring devices, the problem of a wide dynamic range of the return signals have been addressed by ignoring measurements for which the strength of the return signal is above a first threshold or below a second threshold. However, this approach implies unnecessary processing of invalid measurements. Another approach, usually used in so called “rising edge” measurements, is to accept a certain distance measurement error—walking error—due to the large dynamic range, or try to compensate for it, which may be possible to some extent. Yet another approach has been to perform a two-step measurement for each of the target positions of interest. In the first step, a first light pulse, or a series of light pulses, is transmitted towards the target, and the reflected light pulse is detected and processed by the readout electronics to determine an appropriate gain for measurement at the respective position. Typically, in the first step, if the amplitude of the return signal is considered to be low (below a predetermined threshold), the gain is set to a value larger than 1. If the amplitude of the return signal is considered to be high (above a predetermined threshold), the gain is set to a value smaller than 1. Subsequently, in the second step, a second light pulse is emitted towards the target, and the reflected light pulse is detected and amplified using the gain set in the first step. The amplified signal is then processed for determining the distance to the target. In this way, the distance may be measured with an appropriate gain for each of the positions of interest. However, this approach provides a limited measurement rate, and thereby low overall efficiency.
Furthermore, conventional measuring devices directed toward allowing for a high dynamic range in pulses or signals detected at the input of the readout electronics are generally associated with high costs due to need for advanced, custom-designed components. Furthermore, such high-dynamic-range conventional devices generally are not capable of providing fast response times (e.g. of the order of 1 ns or less), which may be necessary for efficiently performing high-speed EDM or scanning (e.g. of such high speed as to enable accurate single shot measurements).
The difficulties in handling a wide dynamic range at the measuring device as discussed in the foregoing pertains to a number of applications, some of which have been mentioned in the foregoing, but may also be relevant for radar applications, etc.
There is thus a need in the art for an improved distance measuring device that addresses one or more of the above problems.